The guarding of large areas against intrusion always presents a considerable problem despite the many systems already developed for accomplishing this purpose. For example, in the case of international borders, the expense of installing an effective system is a major consideration. Another persistent problem stems from the need to make sensitive detection means which do not at the same time deliver excessive numbers of false alarms attributable to spurious stimulations, for instance, by induced fields from power lines or storms, by natural seismic activities, by the presence of wild animals, and by aging and drifting of electrical components forming part of the system.
There are a number of systems which employ an array of elements which must be excited by a specially generated signal, i.e., tuned bridge circuits. There are field radiating systems in which disturbances of the generated field by motion therewithin is detected. There are systems in which transducers detect stress-induced strain using piezoelectric transducers, for example, Laurent, U.S. Pat. No. 3,489,995; McCarty U.S. Pat. No. 2,745,085 and Turner U.S. Pat. No. 3,287,692. Other transducers comprising piezoelectric units are shown, for instance, in Gillespie U.S. Pat. No. 2,477,246; Massa U.S. Pat. No. 2,967,957; Hansell U.S. Pat. No. 2,430,013; Nesh U.S. Pat. No. 3,167,668; and Brunnert U.S. Pat. No. 3,708,702. The present invention is an intrusion detector of the piezoelectric strain-sensing type which seeks to provide an improved signal generating detector configuration for use in an array of individual detector units buried in the earth.